A large study in England has found that smokers trying to quit were substantially more likely to succeed if they used electronic cigarettes than over-the-counter therapies such as nicotine patches or gum. These results offered encouraging but not definitive evidence in the contentious debate about the risks and benefits of these increasingly popular smoking devices.

Researchers interviewed almost 6,000 smokers who had tried to quit on their own without counseling from a health professional. About a fifth of those who said they were using e-cigarettes had stopped smoking at the time of the survey, compared with about a tenth of people who had used patches and gum.

“This will not settle the e-cigarette issue by any means,” said Thomas J. Glynn, a researcher at the American Cancer Society, who was not part of the study, “but it is further evidence that, in a real-world context, e-cigarettes can be a useful, although not revolutionary, tool in helping some smokers to stop.”

The use of e-cigarettes has risen rapidly across Europe and the United States, and regulators are scrambling to figure out how to respond in the absence of hard evidence about their effects. The debate is particularly fierce in the United States, where some experts say the devices could lure children to start smoking, while others contend that they are the best hope in generations to get smokers to switch to something less dangerous than traditional cigarettes.

About 42 million Americans smoke, and some 480,000 people die every year from smoking-related illnesses, one of the United States’ leading causes of preventable death. The central question is whether e-cigarettes will cause the ranks of smokers to shrink or swell.

So far, the evidence is too thin to provide a convincing answer. The Food and Drug Administration has commissioned a broad study, but its results will not be known for years. A clinical trial inNew Zealand, which many researchers regard as the most reliable study to date, found that people given e-cigarettes had only a slightly better quit rate than those with patches. While the long-term health effects of e-cigarettes are unknown, many health experts believe that the concentrations of toxins in the vapor are much lower than in cigarette smoke.

The English study was not a clinical trial, the gold standard of scientific research, in which participants would have been randomly assigned to different groups: for example, one that used e-cigarettes to quit and another that used nicotine replacement therapies. But authors of the study said that they controlled for many factors, including social class, age, level of nicotine dependence and time since the attempt to quit first started. They also said the study, one of the largest to date, offered valuable insights into the real world experiences of smokers.

Prof. Robert West, director of tobacco studies at University College London and senior author of the study, which is to be published Wednesday in the journal Addiction, said that clinical trials could not answer the question most people have about whether e-cigarettes help smokers quit because the devices are changing so fast that they become obsolete before an experiment ends. What is more, he said, people who wanted e-cigarettes and found themselves put in a group that used, say, the patch, would just drop out.

“The medical model is great for cancer drugs, but it doesn’t really work for this situation because there’s nothing to stop participants in the patches group from just going out and buying an e-cigarette,” Professor West said.

He said the database used for the study was financed by Cancer Research UK, a nonprofit group; England’s Department of Health; and several large drug companies that make nicotine replacement therapy, including Pfizer, GlaxoSmithKline and Johnson & Johnson.

Professor West, an e-cigarette optimist, said that solid epidemiological evidence would not be available for at least several decades, while the need for policy decisions is now.

He calculated that more than 5,000 lives could be saved for every million smokers who switched to e-cigarettes, even if the devices carried significant health risks and people used them indefinitely after quitting real cigarettes.

“Potentially millions of lives are at stake, and our job is to help policy makers to protect those lives,” he said in an editorial that accompanied the study.

The standard recommended practice for helping people quit is a prescribed medicine such as Chantix or a combination of nicotine patch and gum together with counseling from a trained professional. But even in England, where these options are widely advertised and nearly free, the majority of smokers rely on willpower or nicotine products bought from a store, and success rates are low. The study found that the rate at which smokers quit by using e-cigarettes was similar to that for people who used prescribed nicotine therapy with brief counseling.

Stanton A. Glantz, a professor of medicine at the University of California, San Francisco, said the study’s limitation was that it tried to measure the effect of e-cigarette use only among smokers who were trying hard to quit, not all smokers.

Professor West responded that the study’s goal was to find out whether e-cigarettes helped people who were trying to stop smoking. But he said data still being analyzed may yet provide clues to the broader effects on smokers generally.

A version of this article appears in print on May 21, 2014, on page A3 of the New York edition with the headline: Study Gives E-Cigarettes Edge in Helping Smokers Quit.